Think back to "Crossover", the first DS9 mirror universe story. When Kira and Bashir see the alliance ships heading for them, the Klingon vessels de-cloak as they approach. Kinda makes a mockery of this whole episode, doesn't it?

Plotline

When Ezri pulls a knife on Quark, he realizes that the residents of the Mirror Universe have come visiting again. She tells him that Grand Nagus Zek is being held by Regent Worf in her universe, and that he will be killed unless Quark supplies a cloaking device. But can the Ferengi survive a universe where double crossing is the norm?